Contact lens fitting is a time consuming and less comfort process. Typically, contact lens fitting involves two major steps: 1) measuring the patient's refraction errors and 2) fitting contact lens for good visual performance and eye comfort. The first step is to place various trial lenses, one by one, in front of the patient's eye and to enable the patient to determine his best corrected visual acuity, which is then written as refraction prescription for eyeglasses or contact lenses. The second step is to put various contact lenses, one at a time, onto the patient's eye and to enable the patient to select the lens with acceptable visual performance and eye comfort. The chances are that the patient may need to go through many lens insertions or even several clinic visits before a pair of acceptable contact lenses can be identified.
Contact lens fitting becomes much more elaborating when to fit multifocal or aspherical contact lenses for presbyopia correction. In addition to every fitting consideration for a conventional contact lens, fitting a multifocal or aspherical contact lens further involves many more parameters and variations, including the added power needed for each eye, the pupil size effect on optical zone selections for added powers, the tolerance of binocular offset in added powers, and the balance between visual acuity for far and near. The chances are that the patient may need to go through too many lenses to try and may decide to give up before a pair of acceptable multifocal lenses is identified.
Auto-refractor and phoropter are commonly used for providing refraction prescription for eyeglass or contact lenses. An auto-refractor is used to produce objective measurement of eye spherical defocus power, cylinder power and cylinder axis. A phoropter is used to refine the measurement of auto-refractor through subjective response from the patients. The auto-refractor and phoropter are each a stand-alone instrument, and they require different sitting and alignment to perform the measurements. Typically it takes 10 to 30 minutes to make a thorough refraction measurement with auto-refractor and phoropter.
An auto-refractor typically measures one eye at a time and directs the subject eye looking at a fixation target inside the instrument. Instrument myopia, due to fixating at an internal target, is commonly an issue to limit measurement accuracy and reliability.
A phoropter is an instrument used to measure refraction status of the eyes. Conventionally, it contains many lenses which are then congaed in front of the eyes while the patient is looking at an eye chart. This is when the doctor usually asks, “which is better, one or two?”
Auto-phoropter is a good advancement from phoropter and is equipped with a control box and motorized mechanism to change trial lenses. Auto-phoropter is basically a motorized phoropter, which employs multiple sets of trial lenses on a plurality of rotational wheels to produce refraction corrections for sphere and astigmatism. The measurement procedure of an auto-phoropter, however, is still elaborating and time consuming.
Besides, a phoropter measures the patient refraction errors referring to a trial lens plane but not on the corneal plane. The nominal distance from the trial lens plane to the corneal plane is typically 12 mm, but the actual distance varies from patient to patient. This actual distance is required to convert accurately the phoropter measurement to prescription for contact lenses, especially for high myopia eyes.
Contact lens simulators or phase plates have been used to evaluate contact lens visual performance without placing a real contact lens on a subject cornea. Contact lens simulators or phase plates are typically made of a thin piece of PMMA and may have similar optical characters of a multifocal or aspherical contact lens when viewing through the lens center. When a contact lens simulator or phase plate is inserted in an eyeglass trial frame, the subject eye may look through the lens center and experience the optical performance of the contact lens in simulation. However, the visual performance test with a contact lens simulator on an eyeglass trail frame is usually less effective, simply because it is placed on an eyeglass plane but not a corneal plane.